The proposed research addresses two questions of central theoretical and practical importance: (1) Why do some young people eventually become addicted to heroin whereas their friends may experiment with the drug but fail to become addicts, and (2) why do other young people, presented with ample opportunities, refuse to try heroin even once? Using a sociological career framework, we view heroin addiction as a process comprised of the following sequential stages: initiation to heroin, occasional experimentation or "chipping," daily heroin use and the onset of physical dependence, maintenance of a heroin habit, and for some, entry into or exit from treatment. By comparing heroin addicts and experimenters with their nonuser friends who fail to try the drug, the project first examines the factors affecting willingness to experiment with heroin. Secondly, by comparing heroin addicts with their experimenter friends who did not become addicts, it describes the factors influencing career outcomes among heroin users. The sample consists of 75 heroin addicts, 75 experimenters, and 75 nonusers; it includes blacks, whites, and Latinos, males and females, from middle- and lower-class economic backgrounds. Addict subjects will be recruited from drug abuse treatment facilities. Experimenters and nonusers will be selected from the addicts' self-defined friendship groups prior to first drug use and will not be members of drug treatment programs. To identify precursor variables that may help to account for differential willingness to try heroin, all subjects will be given an interview which examines demographic characteristics, family backgrounds prior to drug use, life styles and patterns of identification, drug use histories, and attitudes toward heroin. To identify the career contingencies affecting progression from initial heroin use to the onset of physical dependence, interviews with addict and experimenter subjects will also assess the influence of a number of behavioral and attitudinal variables operative at each stage of the addiction process, including definition of the heroin high, frequency and duration of "chipping," availability of heroin, use of other drugs, route of administration, periods of abstinence, occurrence of adverse outcomes, response of family and friends to heroin use, police pressure, and response to physical withdrawal sickness.